Some time ago, an article mentioned that the high current junction box has fully met the requirements of ultra-high power components and presented the data for the 25A junction box. In fact, according to the author's understanding, 25A junction box can not meet the use requirements of super-large current double-sided components. This paper will give a comprehensive analysis from the rated current standard of junction box, the requirement check of high current junction box and reliability and other related issues.
(Source: wechat official account "PV News" ID: PV-News)
1. Selection standard of rated current of junction box
In accordance with the IEC61215 bypass diode test provisions, the rated current of the junction box should meet the Isc(single-sided component) greater than 1.25 times, if applied to the two-sided component, it is also necessary to consider the UL1703 regulation of the two-sided gain factor 30%, so the theoretical maximum current of the double-sided junction box is equal to ISCX 1.25x1.3. Therefore, based on the calculation of 18.36A short-circuit current of G12 double-sided components, the maximum theoretical current of the junction box is equal to 29.84A, and the junction box with rated current of 30A should be selected. According to short-circuit current calculation, the maximum theoretical current of M10 double-sided components is 22.5a, and A junction box with 25A rated current should be selected. According to the above calculation, it is obvious that the safety margin left by the 30A junction box is only 0.5%, which means that the subsequent improvement of the component current through other means (such as battery efficiency improvement, new component efficiency improvement technology) is restricted, and the reliability is at risk if there is a slight fluctuation. Considering the current standard 10% safety margin design, it is even necessary to develop junction boxes with more than 33A. For the M10 component, the safety margin of the junction box is up to 10%, which leaves sufficient space for the subsequent application of new technologies. It is a scientific consideration considering both reliability and long-term development.
2. Inventory of high-current junction box is required
The above table shows the typical electrical performance parameters of high-current modules and the selection of rated current of junction boxes in photovoltaic module factories. It can be seen that: the M10 module with 13.9A current adopts the mature single-core scheme in the industry and has sufficient safety margin. At the same time, higher-specification diodes are used to ensure the reliability of high-current junction boxes. The 18.4A G12 ultra large current module adopts A new dual-core solution (unproven by the industry) and has A small safety margin for double-sided components.
3. Reliability analysis of high-current junction box
According to the survey, most of the junction boxes of M10 components are the mature axial process and single core design of the industry. The shaft diode junction box developed early, and is now relatively mature and has a low failure rate. Now most of the groove chip structure, small leakage current, strong antistatic ability, its reliability has been fully verified. For the design of 30A junction box with dual core, the industry has not used, if the performance of the two chips is very different when the chip is packaged, there will be the risk of uneven shunt at work, resulting in diode heating and burning.
The shunt characteristics of parallel diodes are analyzed as shown in the figure below. The forward current curve of 150mil Schottky diode is used for analysis. If the forward voltage drop of the two diodes is VF= 50mV, the current difference between the two diodes is 10A. Assuming an operating current of 30A, one of the diodes will have a current of 20A, which is close to the upper limit of the diode's allowed current (22A, where junction temperature exceeds 200°C) and the risk of failure is significantly increased.
As junction temperature performance is crucial for junction box selection, the Isc of M10 double-sided assembly is much lower than the current of 18.4a double-sided assembly, and the junction box with the same configuration has lower heat, which can easily pass junction temperature test under conventional conditions. The test data are shown in the following table. For a more comprehensive evaluation of the junction box, the junction temperature performance was tested as follows:
First, the maximum theoretical current of 22.5a was further increased to 26A, and then other tests under the same conditions were carried out. After one hour, the temperature of the diode shell was recorded, and the results were as follows:
It can be seen that even if the test current exceeds the rated current of the M10 junction box, the diode is still safe and reliable. In order to simulate the influence of outdoor comprehensive aging factors (high temperature and high humidity) on the junction box, PCT48h(high pressure accelerated experimental aging)+ junction temperature test (26A) was carried out on the junction box of M10 component to observe the junction temperature. The results are as follows:
The experimental results show that the M10 component junction box still meets the requirements even if the outdoor junction box is superimposed with ultra-high current in high temperature and humidity environment.
To sum up, high-current junction boxes should be evaluated not only for junction box materials, but also for safety margin of junction box design current, maturity of junction box package structure and performance in harsh environment. The M10 component junction box is safe and reliable after being evaluated in the above aspects through rigorous design, while the G12 super-large current double-sided component junction box obviously does not meet the requirements of use.